Previously, for example, Japanese Patent Laid-Open No. 2007-146785 has disclosed a control apparatus which calculates an amount of air sucked into a combustion chamber using a value detected by an in-cylinder pressure sensor (hereinafter referred to as “CPS”) and determines optimal ignition timing using the calculated amount of air. The CPS detects an in-cylinder pressure as a relative pressure with reference to an intake pipe pressure. Thus, a value detected by the CPS needs to be corrected to its absolute pressure in order to be used for various controls. Using the fact that PVκ (κ is a specific heat ratio) is theoretically constant during a compression stroke after an intake bottom dead center, the above-described conventional control apparatus calculates an error (absolute pressure correction value Pr) of the absolute pressure of the value detected by the CPS by the following equation (1) using Poisson's equation established during the compression stroke. Incidentally, in the following equation (1), Pc(θa) and Pc(θb) are values detected by the CPS at two predetermined crank angles θa and θb during the compression stroke, and V(θa) and V(θb) are in-cylinder (combustion chamber) volumes when Pc(θa) and Pc(θb) are detected.Absolute pressure correction value Pr=(Pc(θb)·Vκ(θb)−Pc(θa)·Vκ(θa))/(Vκ(θa)−Vκ(θb))  (1)